@Dudica: I don't know Megan Winters posts, but I think it does not help a lot putting a comment like yours to threads of a total different topic. It's quite likely none of the followers of this thread knows her posts, and by this can not agree or disagree with you.

@John Saunders. I think the problems you can expect are not the big issue, because you can take care of them already at coding time. The big problem are the issues you do not expect. Even that you can find them in advance by doing extra testing on XP/2003, you then need to work around them which is extra work. As far as I know MS did not even release a full list of fixed bugs, so we have no chance to avoid all/most problems in advance.

@John Saunders: It's not that I can't do that. In fact this would be an absolutly possible method. But I want to avoid this additional step if possible. The reason for this is that in our special situation we have to create several extreme stable releases per month. Of course not all modules will be updated on every release, but it's a lot of testing to do. So it would be a one time job for MS to create and release a .Net 4.0 SP1 for XP. But if I use VS2012 without having this SP1 available I have to do a lot of additional testing and debugging without a locally installed VS.
You can call it lacyness. But staying with VS 2010 as long as XP does not get the bugfixes is the way we will go then. In fact we have decided to stop official support for XP the same day as MS does it, which will be Octover 2014.

@Alovchin:
We currently use VS2010 and most likely we will not switch to VS 2012 before XP runs out at our most important customers.
It's just a pitty that we have to use 2010 if we could use 2012 for free (we have a subscription).
If it comes to testing it's not only the OS issue that would cause double testing (which would make sens in a VM). We need to use special harware, which we have in most cases only one time. This hardware does not always allow to run in a VM. Also maintaing two OS on that hardware is work that has to be done. Additionally we have to use Delphi for historical reasons in parallel (it's impossible to port all the software at once). But Delphi has a limited number of installations. You can install it 5 times only, then you need to contact Embarcadero and let them reset it (so far they did this).
I could continue that list of things to do if we need to test everithing twice. Therfore I hope MS will make a ServicePack for .Net 4.0 on XP. If not, I have to wait 2 more years (VS2014 will be out then).
As already mentioned, thats a pitty, because also VS2010 has some bugs which are fixed in 2012 and will never be fixed in 2010 (like the long delay when switching from WPF Designer to a code editor (or similar).

@Alovchin: Unfortuenately it is not my decission to support XP. It's our customers decission. In production environments it's quite common to use older OS very long. You might even find system with NT out there. A Upgrade of a production system costs way more than just a Windows license and eventually a new PC. 5000 to 10000 Euro per system are not unusual. And usually you need to update one by one in production free time (weekends) to make sure production runs smooth.
So finally: we have to support XP for a while (at least two more years). No one would pay for testing everything 2 times (on .NET 4.5 and XP with .NET4.0)
At least my boss allows me to write emails to our customers where we announce to end XP support at the same time as MS (October 2014).